Calibration of measuring sensors and instruments

4.3 Calibration chain and traceability

The calibration facilities provided within the instrumentation department of a company provide the first link in the calibration chain. Instruments used for calibration at this level are known as working standards. As such working standard instruments are kept by the instrumentation department of a company solely for calibration duties, and for no other purpose, then it can be assumed that they will maintain their accuracy over a reasonable period of time because use-related deterioration in accuracy is largely eliminated. However, over the longer term, the characteristics of even such standard instruments will drift, mainly due to ageing effects in components within them. There[1]fore, over this longer term, a programme must be instituted for calibrating working standard instruments at appropriate intervals of time against instruments of yet higher accuracy. The instrument used for calibrating working standard instruments is known as a secondary reference standard. This must obviously be a very well-engineered instrument that gives high accuracy and is stabilized against drift in its performance with time. This implies that it will be an expensive instrument to buy. It also requires that the environmental conditions in which it is used be carefully controlled in respect of ambient temperature, humidity etc.

When the working standard instrument has been calibrated by an authorized stan[1]dards laboratory, a calibration certificate will be issued. This will contain at least the following information:

• the identification of the equipment calibrated

• the calibration results obtained

• the measurement uncertainty

• any use limitations on the equipment calibrated

• the date of calibration

• the authority under which the certificate is issued.

The establishment of a company Standards Laboratory to provide a calibration facility of the required quality is economically viable only in the case of very large companies where large numbers of instruments need to be calibrated across several factories. In the case of small to medium size companies, the cost of buying and maintaining such equipment is not justified. Instead, they would normally use the calibration service provided by various companies that specialize in offering a Standards Laboratory. What these specialist calibration companies effectively do is to share out the high cost of providing this highly accurate but infrequently used calibration service over a large number of companies. Such Standards Laboratories are closely monitored by National Standards Organizations.

 In the United Kingdom, the appropriate National Standards Organization for vali[1]dating Standards Laboratories is the National Physical Laboratory (in the United States of America, the equivalent body is the National Bureau of Standards). This has estab[1]lished a National Measurement Accreditation Service (NAMAS) that monitors both instrument calibration and mechanical testing laboratories. The formal structure for accrediting instrument calibration Standards Laboratories is known as the British Cali[1]bration Service (BCS), and that for accrediting testing facilities is known as the National Testing Laboratory Accreditation Scheme (NATLAS).

Although each country has its own structure for the maintenance of standards, each of these different frameworks tends to be equivalent in its effect. To achieve confidence in the goods and services that move across national boundaries, international agreements have established the equivalence of the different accreditation schemes in existence. As a result, NAMAS and the similar schemes operated by France, Germany, Italy, the USA, Australia and New Zealand enjoy mutual recognition.

The British Calibration Service lays down strict conditions that a Standards Labora[1]tory has to meet before it is approved. These conditions control laboratory management, environment, equipment and documentation. The person appointed as head of the labor[1]atory must be suitably qualified, and independence of operation of the laboratory must be guaranteed. The management structure must be such that any pressure to rush or skip calibration procedures for production reasons can be resisted. As far as the laboratory environment is concerned, proper temperature and humidity control must be provided, and high standards of cleanliness and housekeeping must be maintained. All equip[1]ment used for calibration purposes must be maintained to reference standards, and supported by calibration certificates that establish this traceability. Finally, full docu[1]mentation must be maintained. This should describe all calibration procedures, maintain an index system for recalibration of equipment, and include a full inventory of appar[1]atus and traceability schedules. Having met these conditions, a Standards Laboratory becomes an accredited laboratory for providing calibration services and issuing calibra[1]tion certificates. This accreditation is reviewed at approximately 12 monthly intervals to ensure that the laboratory is continuing to satisfy the conditions for approval laid down.

Primary reference standards, as listed in Table 2.1, describe the highest level of accuracy that is achievable in the measurement of any particular physical quantity. All items of equipment used in Standards Laboratories as secondary reference standards have to be calibrated themselves against primary reference standards at appropriate intervals of time. This procedure is acknowledged by the issue of a calibration certifi[1]cate in the standard way. National Standards Organizations maintain suitable facilities for this calibration, which in the case of the United Kingdom are at the National Phys[1]ical Laboratory. The equivalent National Standards Organization in the United States of America is the National Bureau of Standards. In certain cases, such primary reference standards can be located outside National Standards Organizations. For instance, the primary reference standard for dimension measurement is defined by the wavelength of the orange–red line of krypton light, and it can therefore be realized in any laboratory equipped with an interferometer. In certain cases (e.g. the measurement of viscosity), such primary reference standards are not available and reference standards for calibra[1]tion are achieved by collaboration between several National Standards Organizations who perform measurements on identical samples under controlled conditions (ISO 5725, 1998).

 What has emerged from the foregoing discussion is that calibration has a chain[1]like structure in which every instrument in the chain is calibrated against a more accurate instrument immediately above it in the chain, as shown in Figure 4.1. All of the elements in the calibration chain must be known so that the calibration of process instruments at the bottom of the chain is traceable to the fundamental measurement standards. This knowledge of the full chain of instruments involved in the calibration procedure is known as traceability, and is specified as a mandatory requirement in satisfying the BS EN ISO 9000 standard. Documentation must exist that shows that

process instruments are calibrated by standard instruments that are linked by a chain of increasing accuracy back to national reference standards. There must be clear evidence to show that there is no break in this chain.

 To illustrate a typical calibration chain, consider the calibration of micrometers (Figure 4.2). A typical shop floor micrometer has an uncertainty (inaccuracy) of less than 1 in 104. These would normally be calibrated in the instrumentation department or Standards Laboratory of a company against laboratory standard gauge blocks with a typical uncertainty of less than 1 in 105. A specialist calibration service company would provide facilities for calibrating these laboratory standard gauge blocks against reference-grade gauge blocks with a typical uncertainty of less than 1 in 106. More accurate calibration equipment still is provided by National Standards Organizations. The National Physical Laboratory maintains two sets of standards for this type of calibration, a working standard and a primary standard. Spectral lamps are used to provide a working reference standard with an uncertainty of less than 1 in 107. The primary standard is provided by an iodine-stabilized Helium–Neon laser that has a specified uncertainty of less than 1 in 109. All of the links in this calibration chain must be shown in any documentation that describes the use of micrometers in making quality-related measurements.